Turbulence in deep radio occultations

We investigate here the effect of turbulence in the focal region of spherical or weakly oblate refracting bodies by establishing weak scattering results for the scintillation index and power spectrum. If the spacecraft is a distance r ⊥ from the focus in the plane of the sky, we find that initial focusing alone yields an increase in scintillation index proportional to r ⊥ −5/6 . Still closer to the focus, where the semimajor axis of the atmospheric Fresnel zone has exceeded the outer scale of turbulence L 0 , the scintillation index reaches a plateau. In this regime of maximum scintillations, turbulence of the strength found above the earth's boundary layer, when scaled to the conditions of the focal evolute crossing during the recent Voyager 1/Jupiter encounter, yields a formal weak scattering scintillation index of ≃ 6 · 10 1 . Such strong (saturated) scintillations may have contributed to the unsuccessful attempt to identify the evolute crossing in this case. An approximate calculation also shows that for reasonable solar‐spacecraft distances and probing wavelengths, irregularities in the solar corona plasma strongly affect both the maximum intensification and the signal coherence close to the gravitational focus of the sun. Use of extremely small wavelengths (≤10 −4 m) seems necessary to achieve near optimal conditions for practically achievable solar‐spacecraft distances. The temporal power spectrum of the intensity scintillations is found to be band‐limited close to a focus, with both the bandwidth and the lower frequency boundary increasing rapidly as the focus is approached.